1. Field of the Invention
The present invention relates to techniques and processes for removing low concentrations of organic pollutants and bioaerosols from air. More particularly, this invention relates to processes for efficiently removing organic pollutants (e.g., solvent vapors) and bioaerosols from air or other gaseous streams using a photocatalyst.
2. Description of the Prior Art
Conventional technology is very effective in removing particles from air. However, conventional technology is not able to remove solvents such as alcohols, ketones, halogenated ethylenes, halogenated ethanes, halogenated methanes, surfactants, photoresist solvents, etc. which are used in large quantities for cleaning and other manufacturing processes in all manufacturing environments, for example, a clean room environment. In a clean room, besides being hazardous to worker's health, these chemicals are able to deposit from the air onto "clean" high energy surfaces such as ceramic substrates, semiconductor surfaces, and metal surfaces. Whenever such chemicals deposit on such surfaces, they contaminate the surfaces. This can, and often does, result in ruined materials which must be scrapped if they cannot be readily cleaned. Organic pollutants and bioaerosols also affect the health and comfort of the occupants of commercial and residential buildings.
U.S. Pat. No. 4,666,478 (Boissinot) describes scrubber apparatus for purifying air during embalming or an autopsy. The apparatus includes a housing above a work table, and an air filter comprising aluminum oxide and potassium permanganate is positioned in the housing. An ultraviolet ray tube may be included to kill germs and bacteria. No photocatalyst is described.
U.S. Pat. No. 4,863,608 (Kawai et al.) describes photocatalytic treatment of water contaminated with organic material. The water is first filtered and ultrafiltered, after which the water is irradiated with UV light in the presence of selected inorganic semiconductor and a noble metal or oxide thereof to decompose organic impurities. No Description is made of removal of solvents from air.
U.S. Pat. No. 4,750,917 (Fujii et al.) discloses techniques for cleaning air by irradiating it with UV rays to electrically charge particles in the air and then removing the particles with electrostatic filters. There is no disclosure of the use of a photocatalyst.
U.S. Pat. No. 5,152,814 (Nelson) discloses an air purifier comprising a pre-filter. Then the air is exposed to UV light and passed through a HEPA filter to kill microbes.
U.S. Pat. No. 5,225,167 (Wetzel) describes a process for sterilizing air by trapping airborne particulates in a HEPA filter. An ultraviolet lamp destroys organisms on the trapped particulates. No description is made of a photocatalyst.
U.S. Pat. No. 4,774,026 (Kitamori) describes a process for oxidizing or reducing a substance dissolved in a solution. The process involves adding a particulate semiconductor photocatalyst and a water-soluble electron acceptor or donor to the solution and irradiating it with an electromagnetic wave. There is no description of a process for removing volatile organic compounds from air.
U.S. Pat. No. 5,122,165 (Wang) describes a process for removal of volatile compounds from a liquid. The process requires a combination of pH adjustment, chemical reaction, UV reaction, gas stripping, scrubbing, adsorption and regeneration. There is no description of a process for removing contaminants from air.
U.S. Pat. No. 4,788,038 (Matsunaga) describes a process for killing cells. It does not describe a process for removing contaminants from air.
U.S. Pat. No. 4,264,421 (Bard) describes photocatalytic methods for preparing metallized powders. U.S. Pat. No. 4,303,486 (Bard) describes methods for photocatalytic decarboxylation of saturated carboxylic acids.
PCT WO 91/04094 (Raupp) describes methods employing gas-solid heterogeneous photocatalysis for treatment of groundwater and air polluted by volatile organic compounds and nonvolatile organic compounds. There is no description of efficiently removing volatile organic contaminants from air.
European Patent Application Publication No. 0306301 (Henderson) describes a purifier for water or air to remove organic pollutants.
U.S. Pat. No. 4,544,470 (Herrick) describes a photoelectrochemical structure for chemical reduction-oxidation reactions.
U.S. Pat. No. 4,861,484 (Lichtin) describes a catalytic process for controlled degradation of organic materials in a water-containing fluid at ambient temperature. There is no description of the removal of volatile organic compounds from air.
U.S. Pat. Nos. 4,888,101 and 5,118,422 (Cooper) describe a system for photocatalytically modifying a chemical composition and water purification. The system includes semiconductor powder dispersed and trapped in a layer of glass wool.
U.S. Pat. Nos. 4,892,712 and 5,032,241 (Robertson) describe a device for purifying water or air. The device includes a high surface area matrix with photoreactive metal semiconductor bonded to the matrix.
U.S. Pat. No. 4,943,357 (Van Antwerp) describes a method of photodegrading a metallic chelate complex in an aqueous solution. There is no disclosure of a method for removing organic contaminants from air.
U.S. Pat. No. 4,954,465 (Kawashima) describes photocatalytic apparatus for absorbing odors and decomposing the material.
U.S. Pat. No. 5,035,784 (Anderson) describes a process for degrading complex organic molecules by photocatalysis.
U.S. Pat. No. 5,069,885 (Ritchie) describes apparatus for purification of a fluid such as water using a photoreactive semiconductor.
U.S. Pat. No. 5,126,111 (Al-Ekabi) describes a method of removing, reducing, or detoxifying organic pollutants from water or air with a photoreactive metal semiconductor material, while simultaneously contacting the photoreactive material with a substance that accepts electrons.
U.S. Pat. No. 5,137,607 (Anderson) describes a reaction vessel for photochemical reactions. The vessel includes a light source and a counter-electrode. Provision is made for applying an electrical bias.
U.S. Pat. No. 5,144,146 (Wekhof) describes a method for destroying toxic substances using UV radiation. No description is made of a photocatalyst.
U.S. Pat. No. 5,174,877 (Cooper) describes apparatus for photocatalytic treatment of liquids. No description is made of treatment of air.
PCT Patent Publication WO 91/09823 (Lichtin) describes a photocatalytic process for controlled degradation of vaporized organic materials. The process employs solid catalyst, water vapor, and molecular oxygen.